1. Technical Field
The present invention relates to a MEMS resonator and a manufacturing method of the same.
2. Related Art
In recent years, microelectromechanical systems (MEMS) have exhibited a favorable growth in the usage thereof for apparatuses such as acceleration sensors and video devices. There are various interpretations as to what MEMS include conceptually. While in some cases, it is also referred to as “micro machine”, or “micro system technology (MST)”, MEMS generally mean “fine functional devices produced using semiconductor manufacturing techniques”. Those devices are manufactured based on fine processing techniques developed for fabrication of semiconductors. Currently, MEMS are manufactured independently from other manufacturing processes, or, produced onto an integrated circuit (IC) in a process after completing the manufacturing of IC. The field of applications of MEMS includes electric appliances and automobiles, and the field is still expanding. Processes for manufacturing MEMS have been modified based on common microfabrication techniques of semiconductors. For example, capacitive pressure sensors are known, including a diaphragm that is formed on the same semiconductor substrate concurrently to the formation of a gate of an active element. Refer to JP-A-2004-526299 as an example. Moreover, it is known that in order to make a pressure sensor which combines within a semiconductor device smaller, with the higher functionality and reliability, a conductive layer included in an electric circuit is used for forming a pressure detection unit included in a pressure sensor. Refer to JP-A-2006-126182 as an example.
In JP-A-2004-526299 however, only a static capacitive MEMS structure and a complementary metal oxide semiconductor (COMS) circuit are formed concurrently. In JP-A-2006-126182, although a MEMS structure, a COMS circuit, and an oxide-nitride-oxide (ONO) capacitor are formed on a single chip, the MEMS structure is formed in an interconnection layer, while the lower electrode of the ONO capacitor uses a diffusion layer of a silicon substrate. That is to say, the three devices (CMOS circuit, ONO capacitor, and MEMS structure) have not yet been formed concurrently, while two of the three (CMOS circuit and ONO capacitor, or, MEMS structure and CMOS circuit) have been. The above resulted in the following problems. If the ONO capacitor is not included and therefore not used, limitations are imposed on the structure (less variations) of the CMOS circuits such as analog-digital conversion circuit, and other circuits requiring capacitor other than substrate electrode. Moreover, a system in package (SIP) structure in which the ONO capacitor is packaged in a separate chip results in problems such as increased number of processes, increased cost, and noises generated by a wire bonding of interconnections. If the MEMS structure is not included, it results in the aforementioned problems such as increased noise. Further, the MEMS are processed incrementally in pre/post process. This causes an increase in the number of processes and costs, since the processing steps cannot be carried out concurrently.